The use of mobile communications networks has increased over the last decade. Operators of the mobile communications networks have increased the number of base stations in order to meet an increased demand for service by users of the mobile communications networks. The operators of the mobile communications network wish to reduce the running costs of the base station. One option to do this is to implement a radio system as an antenna-embedded radio forming an active antenna array. Many of the components of the antenna-embedded radio may be implemented on one or more chips.
Nowadays active antenna arrays are used in the field of mobile communications systems in order to reduce power transmitted to a handset of a customer and thereby increase the efficiency of the base transceiver station i.e. the radio station. The radio station typically comprises a plurality of antenna elements, i.e. an antenna array adapted for transceiving a payload signal. Typically the radio station comprises a plurality of transmit paths and receive paths. Each of the transmit paths and receive paths are terminated by one of the antenna elements. The plurality of the antenna elements used in the radio station typically allows steering of a beam transmitted by the antenna array. The steering of the beam includes but is not limited to at least one of: detection of direction of arrival (DOA), beam forming, down tilting and beam diversity. These techniques of beam steering are well-known in the art.
The active antenna array or active antenna system is typically mounted on a mast or tower. The active antenna array is coupled to the base transceiver station (BTS) by means of a fibre optics cable and a power cable. The base transceiver station is coupled to a fixed line telecommunications network operated by one or more operators.
Equipment at the base of the mast as well as the active antenna array mounted on the mast is configured to transmit and receive radio signals within limits set by communication standards.
The code sharing and time division strategies as well as the beam steering rely on the radio station and the active antenna array to transmit and receive within limits set by communication standards. The communications standards typically provide a plurality of channels or frequency bands useable for an uplink communication from the handset to the radio station as well as for a downlink communication from the radio station to the subscriber device.
For example, the communication standard “Global System for Mobile Communications (GSM)” for mobile communications uses different frequencies in different regions. In North America, GSM operates on the primary mobile communication bands 850 MHz and 1900 MHz. In Europe, Middle East and Asia most of the providers use 900 MHz and 1800 MHz bands.
As technology evolves, the operators have expressed a desire for an active antenna array which is able to utilise the existing base-station investments, in addition to providing a new system/band. For example, in the roll-out of long term evolution (LTE) at 700 MHz (US) or 800 MHz (EU), the operators would like to deploy a single antenna at the masthead which could transmit the existing 900 MHz (EU) or 850 MHz (US) GSM signals, using equipment at the base of the mast, as well as providing active antenna functionality for the new LTE installation.
One solution comprises using a dual-band or broadband passive antenna, with two traditional base transceiver stations at the foot of the mast. For example, the dual-band or broadband passive antenna would form part of a traditional base transceiver station. This solution however would suffer various drawbacks. Having a dual band or broadband passive antenna would not allow the two frequency bands to have independent downtilt angles. Both bands would need to share the same downtilt and this would be sub-optimal for either one or both of the frequency bands, depending upon the tilt angle chosen.
There are a number of options for allowing the combination of both existing radio signals in a first frequency band, hereafter referred to as the passive signal, emanating from or travelling to a base transceiver station or remote radio head at the bottom of a mast with radio signals, hereafter referred to as the active signal, from a different band, generated (or received) within the active electronics of an active antenna system (see co-pending application . . . ).
All of these options, however, rely upon some form of filtering of the first signals in the first frequency band, prior to combination with the second signals in the second frequency band. The filtering uses bandpass filters, which typically need to be high performance in terms of their roll-off characteristics. It is difficult to manufacture identically performing filters, particularly from a phase/group-delay perspective.
Small differences in the phase or group delay characteristics of the bandpass filters may impact on the beam-shape, tilt angle and/or sidelobe suppression characteristics of the active antenna array.
One option could be to perform some form of trimming of the bandpass filters upon manufacture. Such trimming for low-power filter technologies is however difficult and expensive and is currently unable to yield the accuracy required (which is typically a phase matching accuracy of few degrees).
It would be desirable to enable some form of phase compensation to take place in the active antenna array itself, at high power, to compensate for any errors, however introduced, on the passive signals.